1. Field of the Invention
The invention relates generally to a sealed lash adjuster and a method for adjusting the amount of liquid sealed in the sealed lash adjuster. More specifically, the invention relates to a sealed lash adjuster and a method for adjusting the amount of liquid sealed in the sealed lash adjuster, with which the inner pressure of the lash adjuster is maintained at or lower than a threshold value without increasing the size of the lash adjuster.
2. Description of the Related Art
A lash adjuster, which automatically adjusts a valve clearance between an intake/exhaust valve and a cylinder head of an internal combustion engine to a value at or around zero, has been brought into practical use. With this lash adjuster, noise due to contact between the intake/exhaust valve and the cylinder head is prevented, and the routine adjustment of the valve clearance is no longer necessary. Examples of such lash adjusters include an externally-oiled lash adjuster that uses the engine oil. If an inappropriate amount of engine oil is supplied or deteriorated oil is continuously used in the externally-oiled lash adjuster, more air or foreign matter may be mixed into the oil. As a result, the lash adjuster may fail to function properly. In other words, whether the externally-oiled lash adjuster properly functions depends on whether the amount and condition of the engine oil supplied thereto are controlled appropriately. However, with a sealed lash adjuster in which liquid such as oil is sealed, the above-described factor responsible for a possible functional failure may be removed. For example, Japanese Utility Model Application No. 01-124008 proposes a sealed lash adjuster described below.
The sealed lash adjuster described in Japanese Utility Model Application No. 01-124008 contains magnetic fluid. In addition, the sealed lash adjuster is provided with a magnet serving as sealing means for sealing a gap between the sliding face of a plunger and the body of the lash adjuster. In the sealed lash adjuster, leakage of the magnetic fluid through the gap is prevented using a phenomenon in which the viscosity of fluid significantly increases in a magnetic field formed by a magnet. As a result, the fluid is sealed in the lash adjuster with high reliability.
In the sealed lash adjuster, when the plunger moves downward, the liquid is pushed out of a high-pressure chamber and moves into a reservoir chamber, and the liquid compresses the gas in the reservoir chamber. As a result, the inner pressure increases in the reservoir chamber. Also, when the sealed lash adjuster is used in an internal combustion engine, the temperatures of the liquid and the gas in the lash adjuster increase due to heat transferred from the internal combustion engine. Such increases also contribute to increases in the inner pressure.
Because increases in the inner pressure promote wearing-out of the sealing means, the sealing means needs to have high wear resistance. However, providing the sealing means with high wear resistance increases the production cost of the lash adjuster. In addition, because increases in the inner pressure increase the amount of gas dissolved in liquid, mixing of the gas into the liquid is further promoted. As a result, the sealed lash adjuster may fail to function properly. Further, as the inner pressure increases, friction increases in a valve system because an unnecessarily strong force is applied, due to such increased inner pressure, between the sealed lash adjuster and a rocker arm, between the rocker arm and a cam, between the rocker arm and an intake/exhaust valve, etc. As a result, wearing-out of a sliding portion may be promoted.
Meanwhile, the size of the sealed lash adjuster is restricted because the sealed lash adjuster shares a limited space with components such as the valve system and a cylinder head. Therefore, the sealed lash adjuster should be as compact as possible. The compact lash adjuster also provides high degree of flexibility in design of the entire portion near the lash adjuster, including the components described above. However, the inner pressure and the size of the lash adjuster are not described in Japanese Utility Model Application No. 01-124008.